1. Field of the Invention
The present invention relates to a pressure detecting apparatus for computing pressure by measuring a displacement of a diaphragm which changes depending on the pressure, and more specifically to a capacitance type pressure detecting apparatus for computing the pressure according to the displacement of the diaphragm detected based on the change in the capacitance formed between the diaphragm and an electrode.
2. Description of the Related Art
A conventional capacitance-based pressure detecting apparatus is described in the Japanese Patent Publication No. 06-1228 (Tokko-hei No.06-1228). FIG. 1 shows a pressure detection unit 1 of the pressure detecting apparatus.
The pressure detection unit 1 comprises a diaphragm 2 made of silicon, first conductive plates 3a and 3b, insulator plates 4a and 4b, supporters 5a and 5b, glass joints 6a and 6b, and second conductive plates 7a and 7b. The first conductive plates 3a and 3b are provided opposite each other at a predetermined distance (d) from the diaphragm 2. The insulator plates 4a and 4b hold the first conductive plates 3a and 3b and the supporters 5a and 5b as insulated from the second conductive plates 7a and 7b. The diaphragm 2 is fixed to the supporters 5a and 5b through the glass joints 6a and 6b.
As shown in FIG. 1, pressure-introducing tubes 8a and 8b penetrate the first conductive plates 3a and 3b, insulator plates 4a and 4b, and second conductive plates 7a and 7b. The pressure-introducing tubes 8a and 8b introduce pressures P1 and P2 respectively to the center of the pressure detection unit 1. A conductive film 9 is provided on the inner surfaces of the pressure-introducing tubes 8a and 8b. The second conductive plates 7a and 7b are electrically connected to the first conductive plates 3a and 3b through the conductive film 9.
With the configuration of the pressure detection unit 1, the left and right portions of the diaphragm 2 function as capacitors having capacitances C1 and C2 respectively.
FIG. 2 shows an example of a pressure detector realized by the pressure detection unit 1. Incompressible fluid (a pressure conducting medium) such as silicon oil, etc. is sealed in portions 10 and 11 or pressure-receiving chambers 14a and 14b in a pressure detector 12. In the incompressible fluid, the pressure detection unit 1 is fixed to the case of the body at the pressure-introducing tube 8a side. When pressure is detected by the pressure detector, it is transmitted from seal diaphragms 13a and 13b to the diaphragm 2 through the incompressible fluid.
However, when the pressure detector is filled with the incompressible fluid such as silicon oil as described above, the capacitances C1 and C2 depend on the dielectric constant. Therefore, if the dielectric constant alters with a change in temperature or pressure, there arises a problem of error in measuring a pressure.
A method of amending the error can be to determine the pressure from a displacement .DELTA. of the diaphragm 2 after assigning the measured values of the capacitances C1 and C2 to the following equation (1), EQU (C1-C2)/{(C1+C2)-2Ck}=.DELTA./d (1)
where d indicates the space between the diaphragm 2 and the first conductive plates 3a and 3b, and Ck indicates the parasitic capacitance generated between the conductors other than the electrodes. PA1 where Ts, Tr, and Tb are determined by the following equation (6). ##EQU3## where Cp indicates the parasitic capacitance based on the stray capacitance, and Rf indicates the feedback resistance of a transmitter used in the measurement. PA1 .epsilon.: dielectric constant between electrodes PA1 S: area of the electrodes PA1 where the displacement .DELTA.d1 is proportional to the pressure, and therefore, the value of (C1-C2)/(C1+C2) linearly changes with the pressure P as shown in FIG. 4B.
The values of the capacitances C1 and C2 can be represented by the following equation (2) where S indicates the area of the electrode, .epsilon. indicates the relative dielectric constant of the pressure conducting medium (silicon oil), and .epsilon.0 indicates the dielectric constant in a vacuum (electric constant). Obtaining the pressure by the above described equation (1) removes the term relating to the dielectric constant of the pressure conducting medium, thereby successfully computing the change indicated by the diaphragm 2 without the influence of the dielectric constant (permittiuity) of the pressure conducting medium which alters with temperature or pressure. ##EQU1##
However, when a high-precision measurement is required, the following problems arise. That is, when a high-pressure is measured, the entire pressure detection unit 1 receives pressure and is externally compressed, and the space between the diaphragm 2 and the first conductive plates 3a and 3b is reduced with the capacitances C1 and C2 increased. Equation (2) can be represented by the following equation (3) where .delta. indicates the change in the space. ##EQU2## Therefore, equation (1) can then be represented by the following equation (4), and cannot remove .delta. which depends on pressure. EQU (C1-C2)/{(C1+C2)-2Ck}=.DELTA./(d-.delta.) (4)
If the temperature of the pressure conducting medium alters, the space between the diaphragm 2 and the first conductive plates 3a and 3b changes, and subsequently the capacitances C1 and C2 also change. Since not a small error occurs from these changes when pressure is to be detected with high precision, a temperature sensor or a secondary pressure sensor is required to make necessary amendments. In this case, such problems as increasing costs arise with an increasing number of required devices and signal processing circuits, etc.
A relatively simple pressure sensor for solving these problems is described in "Smart Pressure Sensors for Industrial Application, SENSORS June 1995, pp. 32, 33, 48, and 49".
FIG. 3 shows the corresponding pressure sensor.
In FIG. 3, 21 is a reference capacitor, and 22 is a sense capacitor. They are formed on a silicon substrate 23. In this apparatus, test pressure is applied to both sides of the reference capacitor 21 and one side of the sense capacitor 22. To the other side of the sense capacitor 22, a reference pressure is applied. A differential pressure is obtained based on the ratio of the capacitance of the reference capacitor 21 to that of the sense capacitor 22 (R=Cr/Cs where Cr and Cs respectively indicate the capacitances of the reference capacitor 21 and sense capacitor 22).
According to the above mentioned document, the ratio R is computed by the following equation (5). EQU R=Cr/Cs=(Tb-Ts)/(Tb-Tr) (5)
Although the apparatus shown in FIG. 3 can avoid the influence of the parasitic capacitance Cp commonly found in both reference capacitor 21 and sense capacitor 22, there is a problem with this apparatus that it cannot compensate for the effect of the parasitic capacitance and Cs and Cr themselves on the reference capacitor 21 and sense capacitor 22 caused by a change in temperature and pressure.
"Silicon Diaphragm Capacitive Vacuum Sensor, K. Hatanaka et al., Technical Digest of the 13th Sensor Symposium, 1995 pp. 37-40" describes a vacuum sensor in which two pressure sensors are used to sufficiently cover a measurement range. However, this example analyzes the relationship between the pressure applied to a sensor and the output of the sensor for each of the sensors, but does not disclose the technology for measurement covering a wide range with high precision by combining two sensor signals in pressure analysis.
A technology of processing output signals from a plurality of pressure sensors is described in the Japanese Laid-Open Patent Publication (Tokkai-hei) No. 7-209122. This technology definitely and consecutively connects the characteristics curves of individual sensors by applying a weighting function in an intermediate range in which the pressure measurement ranges of two pressure sensors based on different measurement principles overlap each other.
However, in the method disclosed in this publication, the weighting function may weight only the output of the sensor indicating a larger change when the outputs of the sensors change with the influence of a disturbance such as a change in the ambient temperature, etc. As a result, there is the problem that a change in output with such a disturbance cannot be reduced. A similar problem arises when a disturbance affects the output from one sensor only, thereby influencing the specification of the characteristics of the intermediate range as well as the characteristics of the sensor. Thus, according to the above described technology disclosed in this publication, a reduction in the precision of a sensor output cannot be avoided.
Furthermore, the technology also has the following problems.
First, when a sensor signal is amended for temperature, etc., the amendment should be made to each of a plurality of sensor signals and amendment data should be prepared for each sensor, thereby causing an increase in costs. Since processes are always performed using a plurality of sensors, the outputs of all sensors should be always monitored and amended. Thus, the process speed is lowered and the consumption of electric power is increased. In particular, a long time is required to perform both a weighting process in an intermediate range and a process of amending outputs from the weighting process, thereby reducing the entire process speed and increasing the consumption of electric power. Furthermore, when a setting of a pressure measurement range is changed, an appropriate signal should be selected from a plurality of sensors, and the selected sensor signals should be smoothly coupled. In such cases, the processes become complicated and a desired precision cannot be easily guaranteed.
The pressure detecting apparatus detects a change made by a difference in pressure as indicated by a diaphragm by a change in capacitance. The problems with this apparatus are, for example, that the transformation characteristics of a detection signal are non-linear because of the stray capacitance or parasitic capacitance generated in the sensor unit, that a measurement error occurs, etc.
Methods of solving these problems are disclosed by the Japanese Laid-Open Patent Publication (Tokkai-sho) No. 64-71211 and the Japanese Patent Publication (Tokko-hei) No. 4-12813. According to the former, the influence of the parasitic capacitance can be reduced by using the ratio between two capacitances. According to the latter, the influence of the parasitic capacitance can be removed by using a fixed capacitance equivalent to the parasitic capacitance.
However, since the capacitance indicates a change in a quadratic curve in response to a pressure according to the former, an amending process such as a linearizing process, etc. becomes complicated. As a result, it is hard to detect pressure with high precision and the measurement range is limited. This is described below by referring to FIGS. 4A through 4C. FIG. 4A simply shows a capacitor portion, and FIGS. 4B and 4C show the relationship between the pressure and capacitance ratio.
As shown in FIG. 4A, in the apparatus in which a movable electrode ELV is provided for two fixed electrodes ELF, the capacitances C1 and C2 between the electrodes indicate changes such that one capacitance increases while the other decreases, if the movable electrode ELV moves left or right when a change in pressure is applied thereto. Assuming that d indicates the space between the movable electrode ELV and the fixed electrode ELF, the capacitances C1 and C2 can be computed as follows, when the movable electrode ELV indicates a change in portion (displacement) of .DELTA.d as indicated by the broken line shown in FIG. 4A. EQU C1=.epsilon.S/(d1-.DELTA.d1) EQU C2=.epsilon.S/(d2-.DELTA.d2)
According to the method disclosed in the Japanese Laid-Out Patent Publication (Tokkai-sho) No. 58-21104), a change in portion of the movable electrode ELV is computed by the following equation. EQU (C1-C2)/(C1+C2)=.DELTA.d1/d1
In the method disclosed in the Japanese Laid-open Patent Publication No. 64-71211, the relationship between the capacitance ratio and the change in pressure is represented by the following equation. EQU C1/C2=(d1+.DELTA.d1)/(d1-.DELTA.d1)
Although the change .DELTA.d1 is proportional to the pressure, a measurement error occurs because the relationship between the capacitance ratio and the pressure indicates a change in a quadratic curve.
As disclosed in the Japanese Patent Publication No. 4-12813, a parasitic capacitance changes with a change in temperature when the parasitic capacitance is canceled using a fixed capacitance equivalent to the parasitic capacitance. Therefore, it is difficult to eliminate the parasitic capacitance over all temperature ranges using a specific fixed capacitance. Furthermore, an output voltage V is obtained based on the following equation using a power-supply voltage E. EQU V=(C1-C2)/(C1+C2).times.E
In this case, the power-supply voltage E may alter, and any noise on this voltage will affect a detected signal and make it unstable. Such noise may also badly affect digital signal processing.